As the last post said the Weber relies on the accelerator pump to richen things up when going hard onto the throttle. Diaphragms do get old and wrinkly or split. Easy to replace. Some pump strokes are also adjustable to give you a bit more if needed or the pump jet can be changed for a bigger one. Go up one size at a time, maybe find out the number on yours and see what others are using.
Valve adjustment is dependant on the cam and nothing to do with the head. Valve clearance also dependant on the cam. C cam normally 16thou inlet/18thou exhaust. Having a B20 F.I. head on your B18 isn't a good idea as the combustion chamber size is much larger on the B20 head. Not easy to know what CR the combo has either. Yes it will work but the valves are also even bigger on the F.I. head than the standard B20 one, and that needs extra carburation and cam. The Weber is a step down from the SU's in that department. With a B18 both the Weber and SU's will work better with a standard B18 head with a suitable CR, say, about 9.5:1. A K cam works well and you can also up the CR to 10:1 as long as you're happy with going onto premium gas. Not so bad now the prices have come down and your engine will like premium anyway.

(Phil beat me to a part of that. Thanks Phil)